BBC science and environment correspondent David Shukman joined the Canadian Coast Guard research vessel, the Amundsen, as it attempted to make a crossing of the Northwest Passage.

Record summer melting of the sea-ice made this famous waterway that connects the Atlantic with the Pacific fully navigable this year.

MONDAY, 15 OCTOBER - JOURNEY'S END

We've done it. After 600 miles (950km) steaming through the Arctic, we've crossed two time zones, completed a journey that generations of explorers attempted and we're now safely back on dry land.

David with cameraman Rob Magee and producer Mark Georgiou

The ground still sways a little, in our hotel in the tiny coastal settlement of Kugluktuk. And when I put a glass down on the table I find myself worrying that it might slide off. Weirdly, it's a bit too quiet. I even miss the sonar.

In fact, as we leave the Amundsen, I am surprised to feel a lump in my throat. This was home for a very intense, awe-inspiring week spent making one of the world's last great journeys.

We feel the need to mark this achievement. So on our final evening we make presents of the copies of the old explorers' maps - one for Captain Lise Marchand, one for the ship (to be mounted on a wall next a picture of Amundsen himself) and one for chief scientist Jean-Eric Tremblay.

In turn I ask them to sign copies of three books about the Northwest Passage certifying that these volumes have actually made the journey. I'm particularly pleased when Lise Marchand flourishes a large official Coast Guard stamp over the pages. She also gives us ship's tie-pins, which we'll treasure.

We rush around saying goodbye - not easy when so many of the crew and the scientists have helped us in some very challenging circumstances - whether freezing in a Zodiac launch or joining a stunning ride to an iceberg by helicopter.

Ice cruncher

We leave as the Amundsen makes a rendezvous with another Canadian Coast Guard icebreaker, the Louis St Laurent. In an isolated, sheltered bay we draw alongside. This is the only other vessel we've seen for a week and it's odd catching sight of unfamiliar figures on its decks.

Talk about trying to assert sovereignty. Not one flag-waving ship but two. And while the Amundsen will spend the winter researching the Beaufort Sea - another disputed Arctic hot-spot - the Louis St Laurent will head east through the Northwest Passage just in case anyone had any doubts about who was in charge.

The Amundsen meets up with the Louis St Laurent

The three of us - producer Mark Georgiou, cameraman Rob Magee and me - gather for a final photo on board. The helicopter that will carry us ashore is ready.

We're exhausted - it was hard work filming and filing in the cold; but the memories are exhilarating.

The spell-binding night-time sight from the dark, silent bridge of the bow surging through the ice; the shattering echoes of the ice crunching against the hull; the enthusiasm of the researchers struggling in foul conditions; the thought of all those who died trying to find the North West Passage.

Once on dry land I email Jean-Eric. He replies with words that mean a great deal to us. "Today," he writes, "it feels like we parted with some of our own."

SUNDAY, 14 OCTOBER - ARCTIC POLITICS

We've obviously been on board too long. Over breakfast we find ourselves brainstorming solutions to the potentially dangerous scramble for control of the Arctic.

Not many - if any - Tunisians have ventured this way before

We debate how Russia is claiming the North Pole, Canada is asserting its sovereignty over the Northwest Passage and the United States is eyeing vast tracts of territory, too. But by the time of the second cup of coffee, we suddenly realise that an answer has already emerged.

The patrols can be stopped, the gun boats put away, the threatened wars cancelled - and we put on more toast. The reason: we learn that a new, decidedly non-Arctic power has burst on the scene - Tunisia.

On board the Amundsen, there's a young researcher who's originally from Tunis. For reasons I never quite understand, Selima Ben Mustapha gave up studying the warm waters of the Mediterranean to venture into the ice of the Arctic instead.

She reckons - and here's an invitation to correct her - that she is the first Tunisian ever to set foot inside the Arctic Circle.

And so it came about that one day, when this expedition paused for refuelling at the tiny port of Nanisivik, she went ashore and carried with her that potent weapon of any land grab, her national flag.

PR machine

So watch them quake now in Moscow, Ottawa and Washington. At the very least, flags do have the power to irritate.

When Denmark planted a Danish flag two years ago on tiny Hans Island, which lies between Canada and Danish-controlled Greenland, the Canadian government scrambled ministers northwards to pose in the snow.

And when a Russian expedition planted a flag on the sea bed at the North Pole last August, the Canadian government derided the act as a stunt more at home in the Middle Ages; but then promptly ordered more bases, ships and patrols for the Arctic.

Similarly a glimpse of the stars and stripes up here would provoke enough anger to fund a battleship or two.

So, as we sip a last glass of orange juice, we wonder: why not spare everyone a lot of trouble and just acknowledge this utterly unofficial Tunisian claim?

And it goes without saying that Selima should be in charge.

Adapting to change

Since moving to Canada, to her current post at the University of Sherbrooke in Quebec, she's become passionate about the Arctic and its fate.

She worries about the impact of globalisation on the Inuit people and the effect of global warming on the ecosystem.

She's an expert in using satellite images to measure the levels of chlorophyll in the ocean - the more chlorophyll is detected, the more plankton are present, which means the more carbon the oceans can absorb from the atmosphere.

But instead of just sitting at her computer she's now on her second trip north.

I ask, not unreasonably, if she minds the cold. "At first it was difficult," she admits. "But it's fine if you wear the right clothes." Clearly she's learning to adapt. On a trip back home in the summer, she found Tunisia far too hot.

SATURDAY, 13 OCTOBER - RECOGNITION

News of the Nobel peace prize being awarded for work on climate change spreads rapidly through the ship.

For the scientists, toiling in the Arctic to gather data on warming, it's a great morale boost that their subject gets such high profile. To have Al Gore and the UN's Intergovernmental Panel on Climate Change win the award produces an amazed reaction.

"I'm not surprised at Gore getting it after all he's done," one researcher tells me, "but the IPCC - that's fantastic."

The science goes on...

But then I press a little further. How does it look that a campaigner, albeit a very famous one, should share the prize with an organisation whose entire purpose is not to campaign but instead to produce definitive assessments of the most rigorous research? I have in mind the criticism that Al Gore has hyped some climate science.

At that point, one scientist does become a little uneasy.

"It's disturbing," he says, "that Al Gore and the IPCC should be considered on the same footing.

"They deserve the prize for different reasons - Al Gore has raised awareness, and for that I can forgive the exaggerations, but the IPCC has more credibility and its message could be undermined."

Writing this on board, there does indeed seem to be quite a gulf between some campaigners' claims about Arctic melting and what the researchers are actually prepared to conclude.

They all seem convinced the Arctic is going through dramatic change - but perhaps not quite as alarmingly fast as some climate activists might claim.

Different perspectives

And I can't help wondering how campaigners would cope with the demanding science carried out in the harshest conditions on this expedition.

Every two hours, day and night, it's someone's job to wrap up carefully and clamber out into the freezing cold to collect samples of air or water.

Some instruments are positioned on the roof of the bridge where the winds are fiercest and I watch in admiration as this grim task is carried out.

It's another scientist's job to descend every two hours into the deafening engine room to collect water drawn in from the sea.

The chief scientist on board, Jean-Eric Tremblay, explains how all this helps to understand the Arctic food chain.

One approach is to study it from the top down - tracking how polar bears eat seals which eat fish and so on. Another is to take a view from the bottom up - starting with the microscopic phytoplankton which act as the foundation stone of the ecosystem.

So a great deal of effort here is going into measuring the exact levels of the plankton, how the ocean interacts with the atmosphere and then seeing how reality matches the computer forecasts for climate change.

This is a world where hard facts count. No glib slogans here. That climate science is now internationally recognised is welcome; but for most here it's time to find the hat and gloves and venture out to collect another batch of samples, and I admire them for that.

FRIDAY, 12 OCTOBER - THE OLD AND THE NEW

We keep hearing the same story - how stunned people are by the record retreat of the sea ice this summer.

The captain of the Amundsen no less, Lise Marchand, who knows these waters well, tells me how it all looked so different even in the 1970s.

We spend some time on the bridge looking at the charts. Areas never expected to be clear of ice were open this year.

During an interview about our route and the conditions along it, we spring a little surprise to test her reaction.

Producer Mark Georgiou has brought along his copies of the 19th Century maps of the Arctic and I slide them before her - whole chunks of land are missing, even the very channel we're travelling through isn't marked.

She's genuinely astonished. And admiring.

"They were so brave," she says, of the explorers who ventured here not knowing which bays would lead anywhere, which peninsulas were in fact islands, in fact, not having a clue at all in this tangled, snow-blasted geography.

And now this world is changing again. Ice that was permanent is shifting. The Northwest Passage itself may become a regular waterway.

Toxic legacy

Change is a theme gripping this whole boatload of scientists. Over breakfast, I chat to one researcher studying how thoroughly manmade pollutants are reaching into the ecosystem.

The weather patterns mean that while many parts of the world receive pollution, only the Arctic acts as the final toxic dumping ground.

There are beluga whales with dozens of times the usual limit of mercury; fish can be contaminated; the whole food chain can be poisoned.

The scientists are carrying out experiments in the Arctic waters

On the top deck in a fierce wind, another researcher gathers air samples every two hours round the clock to monitor changes in the gas given off by plankton.

Another descends into the din of the engine room to collect water samples fed in from the sea to see how that is changing.

All the time I bump into young scientists, wrapped up against the cold, lugging bottles and flasks and boxes on their way to their labs, or waiting in the wind for their appointed hour to lower one instrument or another into the sea.

And now we're reaching further south, there's less ice. I miss it.

It would stifle the waves which are now growing. There may be a storm coming. A warning goes out to tie down anything loose and many of us reach for the pills which we hope will work.

I try to banish an unwelcome thought that flashes to mind: I had carrot soup for lunch.

THURSDAY, 11 OCTOBER - NOT SO SMALL MATTERS

Catastrophe. I lose my earplugs during the night; so I hear far too much of the sonar chirping out its penetrating signal.

Grab the camera: The polar celebrities have arrived

Invaluable science, I'm sure, but in the darkest hours of sleeplessness I do wonder whether every inch of sea bed needs to be surveyed quite so thoroughly.

Coffee revives me. I'm at last getting to know how the coffee machine works.

There's always a long delay between hitting the buttons and getting anything liquid in your mug and if, impatient as I am this morning, I punch the buttons too often a petulant little message flashes up in French saying, "Look, your choice really is on its way".

And then the simmering modern-day traveller's guilt flares up again - the shame of remembering that the pioneers of this route, struggling to prevent their ships from being crushed in the ice, would have loved to have complained about matters as minor as an annoying little squeak and a delay in getting caffeine.

Early sponsorship

I visit the bridge to check our position. Throughout the night, and all day today, we've been steaming south through the McClintock Channel.

The where? Well, this channel, one of hundreds twisting between the islands of the Canadian Arctic, proved to be a kind of missing link in the quest to discover the Northwest Passage.

And like almost every channel, sound, headland, and island in this vast and wintery landscape, it carries the names of the great British Imperial explorers and their sponsors.

Ross, Parry, Franklin, Melville, Barrow and long lists of royalty dot the map. There's even a Boothia Peninsular, an early example of product placement, being named after the wealthy distiller of Booth's gin who funded an expedition.

So to today's ice-flecked, wind-blasted Arctic feature, the McClintock Channel.

Horror discovery

In the mid 19th Century, Francis McClintock became famous for successfully discovering the first evidence of a terrible failure - the ill-fated Franklin expedition of 1845 which set off with great fanfare and then vanished.

In search of him, McClintock had landed on the then little-explored King William Island to our south.

Enduring the ghastly hardship of hauling sledges over the ice, he and his crew stumbled on a rotting wooden dinghy containing two skeletons.

This horrific sight confirmed what the country had long dreaded hearing - that Sir John Franklin and his 100 men had all perished in one of the most disastrous ventures the Empire had ever launched.

McClintock himself suffered terribly from scurvy.

Polar shoot

I try to picture him this morning with the temperature at minus 10C and the wind at 20 knots. Brutal, even for a short burst of filming. Imagine camping.

And also imagine his reaction to life in the Arctic now.

I get through on the satellite phone to do some radio interviews - the line is clear. And when that icon of the Arctic bursts on the scene - the polar bear - everyone rushes to snatch pictures.

There are two of the animals - healthy, curious, and adored.

What would McClintock have done on the ice? Run away? More likely shoot them. He was hungry.

WEDNESDAY, 10 OCTOBER - ICE-PACK DRILL

I listen to the safety briefing but find my mind wandering, purely in self defence of course - the idea of something going wrong up here is really too horrible to contemplate.

The water temperature, we're told, is right now between plus one and minus two Celsius.

Up here, you have to rescue yourself

Water freezes at minus 1.9. So all around us we see the first flakes of ice forming in the water, and then massing into this winter's pack.

And as for search and rescue, well, we're it. One of this ship's roles is to help others in trouble.

So we go through the drill. We struggle into emergency immersion suits, so suffocatingly tight at the neck that our faces end up contorted. And we clamber inside one of the Amundsen's lifeboats, bright orange, hermetically sealed and utterly claustrophobic.

The first danger in these, says First Officer Dany Marcotte, is seasickness because there isn't much ventilation and if one person starts then everyone will.

Enough said - I've been there. Two years ago I spent a few miserable days on a highly unstable Norwegian whaling ship in the turbulent Arctic and categorically vowed never to go on a ship again, let alone one anywhere near the Arctic.

How does an otherwise serviceable human brain allow that level of forgetfulness? These days at sea should give me time to consider that.

Cold dive

Except that we're far too busy. Each decision to go out filming triggers an immediate consequence: getting ready. There's no popping out on to deck to grab a few shots - it's far too cold.

We learn to allow time to switch from the clothes you'd normally wear in any centrally heated building to stuff better suited to working in a freezer: three layers on the legs, five on top, plus the vast Arctic boots and gloves and a hat - even a few minutes without one on leaves you feeling dizzy.

The big camera decides it's had enough and just shuts down

Of course once I've got all this on, and found the right corridor and the right floor and the right door to reach the right deck, I'm so overheated I'm desperate for a lungful of deep freeze. Oh, and by this stage, the bladder usually has to be ignored.

So, suitably clad, we climb into a Zodiac inflatable to accompany two scientists heading out to measure the underwater turbulence of the ocean.

This means lowering a mini-Darlek contraption into the water and allowing it to sink, taking readings as it does.

Doing anything tricky is impossible with gloves but without them is unbearable. The big camera decides it's had enough and just shuts down. Luckily, the second camera survives.

Wish you were here

At the helm is Chief Officer Michel Dufresne. In 20 years in the Arctic, he tells me he's never seen the ice retreat as it did this summer. What about the cold now? He laughs - this isn't really cold, winter isn't here yet.

I ask the two researchers, Caroline Sevigny and Amelie Janin, whether they wouldn't rather be studying the sea water in somewhere warmer, say, the Bahamas. Not at all, they reply, it's really exciting being in the Arctic.

Cold... but a great place to do science

They're right. It's full of surprises.

As we're eating, the whole ship shakes. We've hit heavy ice. The irony is that we're actually looking for thick ice so that tracking buoys can be planted on the ice-floes to study where they go.

The ship reverses for a run-up and charges ahead three times and then, with a massive lurch, breaks free. Later the ship's captain, Lise Marchand, studies the radar images of the ice ahead and opts for a big night time detour.

I go to a window to watch the drama. But I have a pepper steak on the table and an Arctic appetite. They win.

TUESDAY, 9 OCTOBER - ICE, SURPRISINGLY

I've just tried to shave. Not easy when the ship is barging its way through a carpet of ice. But I'm getting used to listening out for signs of trouble - if the engine note drops and the ship slows down it usually means there's heavy ice ahead and the chance of a sharp knock.

In fact, there were quite a few of those during the night. An incredible array of noises and jolts kept waking me - the rasping of the hull as it scythes through thin ice, the clanking of large chunks against the sides, the boom as we hit a particularly large piece, the vibrations shaking the whole ship.

Winter is kicking in and the sea is starting to freeze over

I visited the bridge in the evening. An eerie, almost mystical sight.

The bridge itself was totally dark, and mostly silent, only a few of the crew exchanging muttered instructions. Ahead, through vast plate-glass windows, was a floodlit vista of Arctic sea.

The intensely bright beams picked out lumps of ice, bright white on the surface and a pale turquoise underwater.

We approached what looked like a giant ribbon of white. There seemed no obvious passage. But the helmsman slowed down, spotted a weak part and nudged the vessel through. Actually, not through but over.

The bow rose up slightly on encountering the ice, and then its weight forced the ice apart and we were crunching and banging out to the clear dark water beyond.

"Isn't the Northwest Passage meant to be clear of ice?" I keep asking.

The Amundsen is well equipped to deal with all the Arctic can throw at it

The answer is first that winter is definitely kicking in and the sea is starting to freeze. Also, although the exceptionally warm summer did melt the ice in the passage, it also melted and dislodged vast areas of older, denser ice which are now drifting into our path.

I think we've just hit one of them, as I write this. The whole ship swayed, making my fingers more inaccurate than usual over the keyboard.

But we're in good company. The ship's crew exudes competence and the scientific teams are full of enthusiasm.

We're a tiny floating outpost of humanity navigating and exploring some incredibly remote waters. And now I can smell breakfast. The crew are mainly French-Canadian - the coffee and croissants should be good.

MONDAY, 8 OCTOBER - RESOLUTE

I feel slightly guilty, I don't mind admitting. I write this sitting in a warm hotel room, well fed and comfortable. But when I look up through double-glazed windows I see the dark grey waters of one of the most notorious, treacherous, alluring sea-lanes anywhere on Earth - the fabled Northwest Passage.

To the untrained eye, these Arctic waters look no different to any others - bleak, choppy, remote. But to anyone whose luggage is weighed down by books about the nightmarish sagas of polar exploration, this is no ordinary channel.

As far back as the 1500s, there were dreams in Europe of finding a shortcut to the riches of Asia, a swift connection between the Atlantic and the Pacific.

Parliament offered a vast reward to the first man to make it through. The Admiralty sent repeated missions to force a route through the ice. Over the centuries, some of the great names of British imperial discovery ventured into this forbidding land, and many never made it back out.

I, of course, just flew here, from London to Ottawa to Iqaluit and now to Resolute, at the eastern end of the Passage, the biggest obstacle being airport security rather than dense pack-ice.

Tougher times

I'm now waiting to board a Canadian Coast Guard vessel, the Amundsen, which will carry me where so many others tried and failed - as a seafarers' shanty puts it, "tracing one warm line through a land so wild and savage".

My head spins with the tales of what those pioneers endured - lost, trapped, their ships crushed, their bodies ruined by scurvy and hunger; Sir John Franklin leading more than a hundred men to their deaths in the ice.

October was a particularly cruel month for them, the inexorable approach of winter locking them vice-like in the ice to be battered in the dark for six long months.

One of the more successful explorers, Robert McClure, recorded that "no pen can tell of the unredeemed loneliness of an October evening in this polar world."

Now we're in the grip of a polar October. A husky, tethered outside our hotel, howls in the wind. On the beach, polar bears are spotted scavenging the remains of a butchered whale.

I step outside. There's dark foreboding in the clouds, flurries of snow skid over the waves. At the very edge of the shore the first sea-ice is forming. It makes the water appear greasy. The waves are too leaden to break cleanly, so they rise and flop against the shingle.

'Des resolute'

In the airport, cameraman Rob Magee calls me over to a display cabinet - there's a battered old food tin from a British expedition here in the 1850s. The word "desolate" applies all too readily to Resolute, and Rob takes to calling it "des res".

These are beautiful, the result of intrepid surveying by Victorian navy teams. But huge areas are blank, key channels are missing.

In those days, they weren't even sure there was a Northwest Passage. Now, we know from satellite pictures that this summer's record melting has cleared the entire channel of ice.

That's why we're here. Every scientist we meet is stunned by the speed of change.

The latest forecast had suggested that this year's scale of melting would not come till 2040. What would the early explorers have made of what's happening? Would they mind that every year it's becoming easier to do what they attempted?

Territorial politics

The Canadian newspapers are suddenly busy with stories of the consequences of the Arctic melting. One front page shows a Russian bomber being escorted away from Canadian airspace.

The town of Resolute is our starting point

The Canadian government keeps announcing new measures to assert its sovereignty. There are debates about Canada's claim to control the Northwest Passage.

Right now, though, we have our own challenges.

The Coast Guard requires us to wear a particularly macho kind of cold-weather boot, packed with insulation and fitted with hardened protection at the toe. Only the Canadians make this kind of thing. Or Nasa.

They keep your feet warm but they are not easy to walk in

The Arctic pioneers would have loved them - or maybe not. Sir John Franklin famously survived by eating his boots, which in those days were made of barely digestible leather, while ours consist of heavy duty plastic.

And wearing these vast boots feels a little like having a sizeable wooden box strapped to each foot.

So, in addition to everything else we have to prepare for, we have a new task - learning to walk.